3Com 4800G Series, Configuration Manual

Page: 637 / 1246

PIM Overview 637
Switchover from RPT to SPT
Initially, multicast traffic flows along an RPT from the RP to the receivers. Because
the RPT is not necessarily the tree that has the shortest path, upon receiving the
first multicast packet along the RPT (by default), or when detecting that the
multicast traffic rate reaches a configurable threshold (if so configured), the
receiver-side DR initiates an RPT-to-SPT switchover process, as follows:
1 First, the receiver-side DR sends an (S, G) join message hop by hop to the multicast
source. When the join message reaches the source-side DR, all the routers on the
path have installed the (S, G) entry in their forwarding table, and thus an SPT
branch is established.
2 Subsequently, the receiver-side DR sends a prune message hop by hop to the RP.
Upon receiving this prune message, the RP forwards it toward the multicast
source, thus to implement RPT-to-SPT switchover.
After the RPT-to-SPT switchover, multicast data can be directly sent from the
source to the receivers. PIM-SM builds SPTs through RPT-to-SPT switchover more
economically than PIM-DM does through the “flood and prune” mechanism.
Assert
PIM-SM uses exactly the same assert mechanism as PIM-DM does. Refer to
“Assert” on page 631.
Introduction to BSR
Admin-scope Regions in
PIM-SM
Division of PIM-SM domains
Typically, a PIM-SM domain contains only one BSR, which is responsible for
advertising RP-set information within the entire PIM-SM domain. The information
for all multicast groups is forwarded within the network scope administered by the
BSR.
To implement refined management and group-specific services, a PIM-SM domain
can be divided into one global scope zone and multiple BSR administratively
scoped regions (BSR admin-scope regions).
Specific to particular multicast groups, the BSR administrative scoping mechanism
effectively lessens the management workload of a single-BSR domain and provides
group-specific services.
Relationship between BSR admin-scope regions and the global scope zone
A better understanding of the global scope zone and BSR admin-scope regions
should be based on two aspects: geographical space and group address range.
1 Geographical space
BSR admin-scope regions are logical regions specific to particular multicast groups,
and each BSR admin-scope region must be geographically independent of every
other one, as shown in Figure 193.